1. Field of the Invention
The present invention relates to an optical wavelength converting apparatus and an optical wavelength converting method for effecting wavelength conversion by sum-frequency mixing light from a semiconductor laser diode (LD) and light from another LD. Particularly, the present invention relates to an optical wavelength converting apparatus for emitting laser light, which is capable of high-speed modulation driving, and can be used as light sources in picture display apparatuses, such as laser displays, light sources for optical recording, light sources for optical measurement, etc., more specifically, light sources in electrophotographic image forming apparatuses, light sources for writing or reading record media, and light sources for communications.
2. Description of the Related Background Art
A so-called sum frequency generating method will be described with reference to FIG. 22. FIG. 22 illustrates an optical wavelength converting apparatus for generating sum frequency light as disclosed in U.S. Pat. No. 5,130,844.
In the case of the sum frequency generating method in which two LD light at wavelengths λ1 and λ2 is introduced into nonlinear optical material to generate light at a wavelength λ3 whose frequency is the sum of frequencies of light at wavelengths λ1 and λ2, a selective range of usable LDs can be widened, as compared with the second-harmonic generation (SHG) method in which light at a wavelengths λ3 is generated using LD light at a wavelength 2 λ3. In FIG. 22, reference numerals 10 and 30 denote laser beams. Reference numerals 11 and 31 denote LDs. Reference numerals 11a and 31a denote end faces of the LDs. Reference numerals 12, 32 and 36 denote rod lenses serving as collimator lenses. Reference numeral 13 denotes a polarization beam splitter. Reference numeral 14 denotes an optical wavelength converting element. Reference numeral 15′ denotes a mirror. Reference numeral 15a′ denotes an end surface of the mirror. Reference numeral 16 denotes a stopper for absorbing light. Reference numeral 20 denotes a reflection grating. Reference numeral 21 denotes a rotational axis. Reference numeral 33 denotes a half mirror. Reference numeral 35 denotes sum frequency light.
The laser beam 10 at the wavelength λ1 is confined between the end face 11a of the LD 11 and the end surface 15a′ of the mirror 15′, and laser-oscillated. The laser beam 30 at the wavelength λ2 is also confined between the end surface 15a′ of the mirror 15′ and the reflection grating 20, and likewise laser-oscillated. There is provided on the end surface 15a′ of the mirror 15′ a coating for reflecting approximately 100% of laser beams 10 and 30 at the wavelengths λ1 and λ2 while transmitting therethrough approximately 100% of the sum frequency light 35 at the wavelengths λ3. In this structure, both the LDs 11 and 31 have external resonator structures. Accordingly, power density of light in the optical wavelength converting element 14 is higher than a case where no external resonator is provided. High conversion efficiency can hence be obtained. As mentioned above, this optical wavelength converting apparatus includes two external resonator structures.
In the optical wavelength converting apparatus as discussed above, however, the physical length of the laser resonator increases since there are provided two external resonator structures. Further, it is quite difficult to phase-match longitudinal modes of those two resonators with the sum frequency light in the wavelength converting element. Therefore, the conversion efficiency of the sum frequency light is liable to easily fluctuate, and its output is likely to be unstable. Thus, the above structure is not suitable for high-speed modulation.